This invention relates to a process and apparatus for coding and transmission of digital signals using data compression techniques. The invention is applicable to coding operations of image signals, such as digital television signals, but is also applicable to audio or other digital signals.
More specifically, the invention relates to coding and transmission operations in which the data compression is achieved using vector quantization.
In fact, data compression techniques have the objective of reducing the bit rate of information to be transmitted by coding which eliminates redundancies in the digital signal to be transmitted while observing a certain constraint of quality for the received signal.
The technique of vector quantization meets this objective by substituting for each component of the digital input signal, represented in the form of a vector of M variables, a representative vector chosen from a pre-defined and limited set of vectors. More exactly, the space formed by the set of possible vectors for the digital input signal is partitioned into a reduced number of cells each grouping a plurality of vectors and therefore a plurality of possible input signal states. All the vectors of the same cell are then systematically represented by single "representative" vector for the cell. The digital signal actually transmitted is thus limited to a succession of representative vectors, preferably each coded by a label i.
This technique thus indeed does achieve compression of the digital input signal, by empoverishing the transmitted information. In the case of transmission of a digital audio or television signal, a big reduction in the information bit rate can be achieved without affecting the psycho-auditive or psycho-visual sensitivty of the receiver.
Of course, the operation of the system requires the receiver to be able to decode the transmitted signal, that is to say recover the characteristics of the representative vector corresponding to each label i received. This decoding function is performed using a look-up table indicating the corresponding label/representative vector called a code-book. To enable real-time decoding of the transmitted signal, the code-book must be previously transmitted to the receiver, before transmission of the useful digital signal.
The performances of these known techniques of coding depend on the way of generating the code-book and, more exactly, on the process of partitioning and then on the choice of the representative for each cell.